An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing angle and capability for full color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. Recently, there is an increasing demand for broadband OLEDs to be incorporated into various applications, such as a solid-state lighting source, color display, or a full color display. By broadband emission, it is meant that an OLED emits sufficiently broad light throughout the visible spectrum so that such light is used in conjunction with filters or color change modules to produce displays with at least two different colors or a full color display. In particular, there is a need for broadband light OLEDs (or broadband OLEDs) where there is substantial emission in the red, green, and blue portions of the spectrum, wherein a broadband-emitting EL layer is used to form a multicolor device in conjunction with filters or color change modules.
A white OLED device is one type of broadband OLED device such that the emission generally has 1931 Commission Internationale d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.31, 0.33). White OLEDs have been reported in the prior art, such as reported by Kido et al. in Applied Physics Letters, 64, 815 (1994), J. Shi et al. in U.S. Pat. No. 5,683,823, Sato et al. in JP 07-142169, Deshpande et al. in Applied Physics Letters, 75, 888 (1999), and Tokito, et al. in Applied Physics Letters, 83, 2459 (2003).
In order to achieve broadband emission from an OLED, more than one type of molecule has to be excited because each type of molecule only emits light with a relatively narrow spectrum under normal conditions. A LEL comprising a host material and one or more than one luminescent dopant(s) can achieve light emission from both the host and the dopant(s) resulting in a broadband emission in the visible spectrum if the energy transfer from the host material to the dopant(s) is incomplete. However, a broadband OLED with only one LEL will have neither a wide enough emission covering the whole visible spectrum, nor will it have a high luminance efficiency. A broadband OLED having two LELs can have better color as well as better luminance efficiency than a device with one LEL. However, it is difficult to achieve a broad emission with a balanced intensity from more than two colors because a broadband OLED having two LELs typically has only two intensive emission peaks. For example, in a commonly used broadband OLED having two LELs, if the colors of the LELs are yellow and greenish blue, the red, green, or blue color emissions will be weak in the device; if the colors of the two LELs are red and greenish blue, the green, yellow, or blue color emissions will be weak in the device; and if the colors of the LELs are green and red, the blue, blue-green, or yellow colors will be weak. A broadband OLED having three LELs of different colors was also proposed but it is still difficult to achieve a broad emission from the device because the most intensive light typically comes from the LEL with a dopant having the narrowest optical band gap and the emission spectrum shifts with different drive conditions.
In a full color display using broadband OLEDs as the pixels, the perceived red, green, or blue color from the human eyes comes from the pixels with a red, green, or blue color filter on top of the pixels, respectively. If each of the broadband OLED pixels in the display has an emission including balanced red, green, and blue primary color components, the light intensity passing through the color filter is about one third of the broadband emission intensity. However, if the broadband OLED pixels do not have balanced red, green, and blue emission, one of the primary color components will have the intensity lower than one third of the broadband emission intensity after passing through the color filter. As a result, in order to achieve a comparable emission intensity of the specific primary color, the corresponding broadband OLED pixel has to be driven with higher current density causing higher power consumption and a shorter lifetime. Therefore, color compensation is needed for a conventional broadband OLED to achieve balanced red, green, and blue emission.
Similar issues arise in a full color display using broadband OLEDs as the pixels with red and green color change modules with or without red, green, or blue color filters. If the broadband OLED pixels in the display have an emission that results in balanced red, green, and blue primary color components after the color change modules or color filters, then the light intensity for each colored pixel is about one third of the total light intensity. However, if the broadband OLED pixels do not have balanced red, green, and blue emission after the color change modules or color filters, then in order to achieve a comparable emission intensity of the specific primary color, the corresponding broadband OLED pixel has to be driven with higher current density, causing higher power consumption and a shorter lifetime. Therefore, color compensation is also needed for a conventional broadband OLED used with color change modules with or without color filters to achieve balanced red, green, and blue emission.
In order to improve the full color emission of an OLED, stacked OLEDs have been fabricated as disclosed by Forrest et al. in U.S. Pat. No. 5,703,436. These stacked OLEDs are fabricated by vertically stacking multiple, individually addressable OLED units, each emitting light of a different color, and wherein intra-electrodes are provided between each of the vertically stacked OLED units as a way of independently controlling the emission from each individual OLED unit in the OLED device. As a result, full color emission as well as a balanced white color emission is readily achieved. Although this permits for improved color emission and a larger emission area compared to conventional full color OLEDs, the overall construction of the OLED is complex, requiring transparent electrodes, additional bus lines for providing electrical power, as well as a separate power source for each of the stacked OLED units.
Recently, another new type of stacked OLED (or tandem OLED, or cascaded OLED) structure used for EL improvement has been fabricated by Jones et al. in U.S. Pat. No. 6,337,492, Tanaka et al. in U.S. Pat. No. 6,107,734, Kido et al. in JP Patent Publication 2003/045676A and in U.S. Patent Publication 2003/0189401 A1, Liao et al. in U.S. Pat. No. 6,717,358 and U.S. Patent Application Publication 2003/0170491 A1, the disclosures of which are herein incorporated by reference. This stacked OLED is fabricated by stacking several individual OLEDs vertically and driven by only a single power source. Matsumoto and Kido et al., reported in SID 03 Digest, 979 (2003) that a tandem white OLED is constructed by connecting a greenish blue EL unit and an orange EL unit in the device, and white light emission is achieved by driving this device with a single power source. Although luminance efficiency is increased, this tandem white OLED device has weaker green and red color components in the spectrum. In U.S. Patent Application Publication 2003/0170491 A1, Liao et al. described a tandem white OLED structure by connecting a red EL unit, a green EL unit, and a blue EL unit in series within the device. When the tandem white OLED is driven by a single power source, white light emission is formed by spectral combination from the red, green, and blue EL units. Although color emission and luminance efficiency are improved, this tandem white OLED cannot be made with less than three EL units, implying that it requires a drive voltage at least 3 times as high as that of a conventional OLED. Another problem of white OLEDs constructed by connecting a red EL unit, a green EL unit, and a blue EL unit in series within a device is that the individual EL units age at different rates, causing a shift in the color of the white OLED over operational time. It is well known that narrow band OLED devices, especially blue and green devices, are typically lower in operational stability than broadband OLED devices. There is a need, therefore, to improve the stability of stacked broadband OLED devices.
A broadband-emitting electroluminescent (EL) layer is used to form a multicolor device. Each pixel is coupled with a color filter element or a color change module element as part of a color filter array (CFA) or a color change module array to achieve a pixilated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element or color change module element. Therefore a multicolor or RGB device is produced without requiring any patterning of the organic EL layers. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340. Other examples of white-light-emitting OLED devices are disclosed in U.S. Pat. No. 5,683,823, JP 07-142169, and U.S. Pat. No. 5,405,709.
One problem in the application of broadband OLED devices, when used with color filters or color change modules, is that the intensity of one or more of the colored components of the emission spectrum is frequently lower than desired. Therefore, passing the broadband light from the OLED through the color filters provides one or more colored light(s) with a lower efficiency than desired. Consequently, the power that is required to produce a white color in the display by mixing red, green, and blue light can also be higher than desired. Therefore, there is a continuing need for improvement in multicolor OLED displays using broadband light-producing OLED devices.
There is also a problem with broadband OLED displays that use one or more EL units that comprise only one LEL within the EL unit. Color compensation of broadband OLED devices that produce at least one color component having an intensity less than desired is achieved by the addition of an EL unit that emits in the wavelength range of the color component having an intensity less than desired. The purpose of this color compensation is to improve efficiency and color purity of the broadband OLED display. It has been observed, however, that narrowband EL units with only one LEL are not as stable as broadband EL units with more than one LEL. Therefore, there is a need to improve broadband OLED devices for color, efficiency, and stability.
Another problem exists with tandem broadband OLED devices due to optical interference effects within the multi-layer OLED structure. It is known in the art that the location of the LEL relative to the reflector layer, typically one of the electrodes, and other interfaces of mismatched optical constants determine the amount of light extracted from the device. The preferred locations for a particular LEL are wavelength dependent. In tandem OLED devices it is difficult to place all of the LELs near their preferred locations. Therefore, there is a need to improve tandem broadband OLED devices to increase the amount of extracted light.
There is also a problem with broadband OLED devices that include emission in non-desired wavelength ranges when used to produce multicolor or full color displays. Power is wasted because a portion of these wavelength ranges are typically absorbed by the color filters used to produce the desired color pixels. In addition, the portions of these non-desired wavelength ranges that are not absorbed result in a decrease in the color purity of the color pixels. One example is a broadband OLED device that includes emission in the cyan, yellow, or magenta wavelength ranges when used to produce full color displays. Power is wasted because portions of these wavelength ranges are typically absorbed by the color filters used to produce the red, green, and blue pixels. In addition the portions of the cyan, yellow, or magenta wavelength ranges that are not absorbed result in a decrease in the color purity of the red, green, and blue pixels. Therefore, there is a need to improve broadband OLED devices to include lower levels of emission in the non-desired wavelength ranges of a multicolor or full color display.